Object.prototype.objCustom = function() {};
Array.prototype.arrCustom = function() {};

let iterable = [3, 5, 7];
iterable.foo = 'hello';

for (let i in iterable) {
  console.log(i); // 0, 1, 2, "foo", "arrCustom", "objCustom"
}

for (let i in iterable) {
  if (iterable.hasOwnProperty(i)) {
    console.log(i,iterable[i]); // 0, 1, 2, "foo"
  }
}

for (let i of iterable) {
  console.log(i); // 3, 5, 7
}